1. Field of the Invention
The present invention generally relates to fabrication of semiconductor devices and more particularly to an electron beam exposure system and method for exposing semiconductor patterns by a charged particle beam such as an electron beam on an object such as a semiconductor wafer.
2. Description of the Related Arts
Electron beam lithography is an indispensable technology for producing advanced semiconductor integrated circuits having a large integration density. With electron beam lithography, it is possible to expose patterns having widths below 0.05 .mu.m with an alignment error below 0.02 .mu.m. Accordingly, electron beam lithography is considered to play a vital role in the production of future semiconductor devices such as DRAMs having a memory capacity exceeding 256 Mbits.
In the production of semiconductor devices by electron beam lithography, the throughput of the production is very important in addition to the resolution of device patterns. Since the exposure is effected using a single converged electron beam in the case of electron beam lithography, electron beam lithography is more disadvantageous in this respect than a conventional optical exposure method which can exposure the entire device pattern in a single shot. However, the resolution by the conventional optical expose system has nearly reached the limit, and for this reason, the electron beam exposure process has to be used for the production of future high speed semiconductor devices or large memory capacity semiconductor memories.
Under such a situation, various attempts have so far been made in order to improve the throughput of the electron beam exposure. For example, the inventor of the present invention previously proposed so-called "block exposure process" and "BAA exposure process." In the block exposure process, a device pattern is divided into a number of basic patterns, and the electron beam is shaped in accordance with such basic patterns. The block exposure method has successfully attained a throughput of about 1 cm.sup.2 /sec at present. By the BAA exposure method, on the other hand, a single electron beam is divided into a large number of electron beam elements arranged in rows and columns, and a large number of exposure dots are formed on a substrate in accordance with the exposure pattern by turning on and off the individual electron beam elements. By the BAA exposure process, it is possible to expose, at a high speed, an exposure pattern having a complicated shape on a substrate.
FIG. 1 is a schematic diagram of a conventional electron beam exposure system that employs the block exposure process.
Referring to FIG. 1, the electron beam exposure system generally comprises an electron optical system 100 for producing a focused electron beam and a control system 200 for controlling the electron optical system 100. The electron optical system 100 includes an electron gun 104 as an electron beam source, wherein the electron gun 104 includes a cathode 101, a grid 102 and an anode 103, and emits an electron beam as a diverging electron beam along a predetermined optical axis.
The electron beam formed produced by the electron gun 104 is passed through a beam shaping aperture 105a provided in an aperture plate 105 for beam shaping. The aperture 105a is formed in alignment with the optical axis O and shapes the incident electron beam into a rectangular sectional shape upon passage therethrough.
The electron beam thus shaped is converted to a parallel electron beam by an electron lens 107 having a focal point at the aperture 105a, and is further focused on a block mask 110 by the electron lens 107b. Thereby, the lens 107b projects the image of the rectangular aperture described above on the block mask 110. As shown in FIGS. 2(A) and 2(B), a large number of apertures 110a corresponding to the basic patterns of the semiconductor devices to be exposed, are formed in the block mask 110, and each of these apertures shapes the electron beam in accordance with the shape thereof.
In the system of FIG. 1, deflectors 111, 112, 113 and 114 are provided so as to select a desired aperture by deflecting the electron beam. Here, the deflector 111 is activated by a control signal SM1 and deflects the electron beam away from the optical axis O, whereas the deflector 112 is activated by a control signal SM2 and deflects the electron beam in an opposite way such that the beam travels in parallel with the optical axis. After passing through the block mask 110, the electron beam is so deflected toward the optical axis O by the deflector 113 activated by a control signal SM3, and the electron beam is further deflected by the deflector 114 activated by a control signal SM4 in such a manner as to travel along the optical axis O. Further, the block mask 110 itself is disposed movably in the direction perpendicular to the optical axis O, so that any one of the beam shaping apertures can be selected over the entire surface of the block mask 110.
After passing through the block mask 110, the electron beam passes through lenses 108 and 116 acting as a demagnifying optical system and is converged on a focal point f.sub.1 on the optical axis O to form the image of the selected aperture on the focal point f.sub.1. The electron beam thus converged passes through a blanking aperture 117a formed in a blanking plate 117 and is then focused on a substrate supported on a movable stage 126 by electron lenses 119, 122 that constitute another demagnifying optical system. The electron lens 122 functions as an objective lens, and includes various correcting coils 120, 121 for correcting focus and aberration as well as deflectors 124, 125 for moving the converged electron beam over the surface of the substrate.
To control the exposure operation, the electron beam exposure system shown in FIG. 1 uses a control system 200. The control system 200 includes storage devices such as a magnetic tape device 201, magnetic disks 202, 203, and so forth, for storing exposure data relating to device patterns of semiconductor devices that are to be exposed. In the case of the example shown, the magnetic tape 201 is used for storing various design parameters, the magnetic disk 202, for storing exposure patterns.
The data stored in the storage devices are read out by a CPU 204 and then transferred to an interface 205 after data decompression. The data stored in the data memory 206 is transferred to a first control unit 207 for generating the control signals SM1 to SM4 described above, and the control unit 207 supplies the data to the deflectors 111 to 114. Further, the control unit 207 generates control signals sent to a mask moving mechanism 209, and the mask moving mechanism 209 moves the block mask perpendicularly to the optical axis O in response to the control signal. Thus, a desired aperture on the mask 110 can be selected by deflecting the electron beam by the deflectors 111 to 114 and further moving the block mask 110 laterally.
Further, the first control unit 207 sends control signals to a blanking controller 210, wherein the blanking controller 210 generates a blanking signal for interrupting the irradiation with the electron beam in response thereto. Next, the blanking signal is converted to an analog signal SB by a D/A converter 211, and the analog signal SB controls the deflector 115 in such a manner that the electron beam deflects away from the optical axis O. As a result, the electron beam is off the blanking aperture 117a and does not reach the surface of the substrate 123.
The control unit 207 further generates pattern correction data H.sub.ADJ and sends the same to a D/A convertor 208, wherein the D/A convertor 208 generates a control signal S.sub.ADJ in response to the pattern correction data H.sub.ADJ and supplies the control signal S.sub.ADJ thus produces to a deflector 106 that is interposed between the electron lenses 107a, 107b. Thereby, the shape of the electron beam passing through the aperture of the mask 110 can be changed. The latter function is used in the case where a desired electron beam shape is different from the shape obtained by the aperture on the block mask 110.
The interface 205 further extracts data for controlling the movement of the electron beam over the surface of the substrate 123 and supplies the data to a second control unit 212. Receiving the data, the second control unit 212 generates a control signal for deflecting the electron beam over the surface of the substrate 123 and supplies the resultant control signal to a wafer deflection control unit 215. The wafer deflection control unit 215, in turn, generates a deflection control signal according to the control signal supplied thereto and supplies the deflection control signal to D/A convertors 216 and 217. The D/A convertors 216, 217 thereby generate driving signals SW1 and SW2 for driving deflectors in accordance with the deflection control signals respectively, and supply these signals to the deflectors 124, 125 for causing the deflection of the electron beam. Further, the position of a stage 126 is measured by a laser interferometer 214. Thereby, the wafer deflection control unit 215 changes the output deflection control signal and hence the driving signals SW1 and SW2, in accordance with the result of measurement of the stage position by the laser interferometer. Further, the second control unit 212 generates a control signal for moving the stage 216 horizontally.
FIG. 2(A) is a perspective view of the structure of the mask 110. It will be seen that a large number of apertures 110a are formed on the mask 110 in accordance with basic patterns. When the electron beam is to passed through one of these masks 110a as shown in FIG. 2(B), the electron beam is shaped in accordance with the shape of the selected aperture 110a, and the electron beam thus shaped is projected onto the substrate 123. In the exposure system shown in FIG. 1, the exposure is turned on and off in the form of shot in accordance with the blanking signal supplied to the electrostatic deflector 115, and one of the apertures 110a on the mask 110 is selected and exposed in each shot.
FIG. 3 shows an example of a BAA mask 110' used for BAA exposure.
Referring to FIG. 3, the mask 110' is used in the system shown in FIG. 1 in place of the mask 110, and a large number of very small apertures 1A.sub.1, 1A.sub.2, 1A.sub.3, . . . are formed in rows and columns. Further, an electrostatic deflector is provided to each aperture. When actuated, each electrostatic deflector deflects the electron beam passing through the corresponding aperture, and as a result, a large number of electron beam elements arranged in rows and columns are formed from the single electron beam generated by the electron gun 104, and an arrangement of exposure dots corresponding to the electron beam elements passing through the apertures of FIG. 3 is projected onto the substrate 123 shown in FIG. 1. When the mask 110' shown in FIG. 3 is used, an arbitrary exposure pattern can be exposed at a high speed on the substrate in the form of shots corresponding to the blanking signals, in the same way as in the case of the block exposure.
In the electron beam exposure system shown in FIG. 1, it should be noted that skip or blur of shot should not occur in each of the shots, whether the exposure is the block exposure or the BAA exposure. On the other hand, it should be noted that the region that is exposed on the substrate in one single shot of the electron beam generally has a size of several microns, and the exposure is executed at a rate of about 200 nsec per region. This means that the exposure clock corresponding to the blanking signal has a frequency of at least 5 MHz, and the total number of shots per substrate exceeds 1 G shots. Since the exposure is carried out under such an extremely stringent condition, it is extremely difficult to completely prevent the occurrence of skips of shot and blur due to erroneous operation of the controllers, discharge of high voltage portions, charge-up of a vacuum column which accommodates the electron optical system, external noise, and so forth. Particularly when the electron beam deforms on the upstream side of the round aperture 117a on the optical axis, the electron beam is cut off by the aperture plate 117 and does not reach the substrate, or the intensity of the electron beam drops even when it reaches the substrate, causing skips of shot or blur. In order to ensure the high reliability of the exposure, it is necessary to inspect the exposure pattern by an inspection instrument such as an optical microscope or an electron microscope, but such inspection is extremely difficult in the case of integrated circuit of a large integration density.
To solve the problem of defective exposure, the Japanese Patent Laid-Open No. 102930/1989 teaches a construction that measures a current corresponding to the electron beam being cut off by the round aperture plate 117 and calculates the number of the shots on the basis of the measured current value. The prior art further compares the value obtained with the number of the shots calculated from the exposure clock or the blanking signal and detects the failure of exposure when there are any differences detected therebetween.
FIG. 4 shows an example where the structure described above is applied to the electron beam exposure system of FIG. 1. In the construction shown, a current generated as a result of the irradiation of the electron beam on the blanking plate 117 is amplified by an amplifier 250, and the resultant output voltage is compared by a comparator 251 with a predetermined reference voltage. The reference voltage is set so that an output pulse signal is produced when the interruption of the electron beam by the blanking plate 117 is complete. Accordingly, when a shot is completely conducted, a pulse signal that corresponds to the blanking signal in one to one relationship is obtained from the comparator 251. When one or more of the shots is incomplete, on the other hand, the output pulse signal from the comparator 251 differs from the blanking signal, and such a difference can be detected by comparing the number of the pulses of the blanking signal with the number of output pulses of the comparator 251.
When such a conventional construction is applied to the electron beam exposure system shown in FIG. 1 that uses the block mask 110 or the BAA mask 110', there occurs the following problem. Because the exposure pattern to be exposed is different in each shot, the amount of the electron beam current cut off by the round aperture 117 changes variously in each shot as shown in FIG. 5 even though the exposure is ideally executed and skip of shot and blur of the shots do not exist at all. Thereby, the detection of the shots by the comparator 251 becomes substantially impossible. In other words, the conventional construction described in the aforementioned reference, i.e. Japanese Patent Laid-Open No. 102930/1989, cannot be applied to the electron beam exposure system for effecting the block exposure or BAA exposure process.
On the other hand, it may be possible in principle to detect the actual number of shots by detecting the current flowing through the substrate 123 and to detect skip of shot and blur by comparing this number with the number of times of the shots obtained from the blanking signals. However, the substrate 123 has a substantial stray capacitance and the capacitance value and its resistance value, too, change with the condition under which the substrate is held. When the need of detecting high speed pulse signals of over hundreds of mega-Hertz are taken into consideration, this method is not practical.